Do cast iron boilers run more effecient at high temperatures?

Cory

I was at a supply house the other day and overheard 3 or 4 installers and distributors talking about running cast boilers at 200F and higher because they ran more effecient. If you use 200f+ as a design boiler supply temp. with injection it seems the boiler would not have to fire as often. I would like to hear some input/opinions on this?

Bill NTSG

No

High temp . does not equal efficiency. In europe the design high limit is 75°C. or 167°F. For every 3°F you can turn back or lower your temp. you should save 1% of your anual energy cost.

Most of the year I heat my house with 130° water. Higher temp. = more energy , less comfort , noisier system. Hey, why have a gas pedal on a car or truck. We'll just set it a full rpm and drive with the brakes and the clutch.

Mike T., Swampeast MO

Maybe

in the first two heat transfers--the flame to the HX and the HX to the water. IB+R ratings seem to demonstrate this.

Some will say that the iron used in certain European boilers of essentially traditional design is of a different (better) grade that has a heat different heat transfer coefficient. Supposedly this allows them to retain their efficiency at lower temperatures. Combined with particular attention to insulation around the HX this supposedly makes these boilers significantly more efficient than their American counterparts.

Should you run a boiler at a higher temperature than required by the system? It all depends. You have to consider the type of device emitting heat to the space; the mass of that device; the volume of tranfer medium (fluid in the pipes); the type of control (digital/proportional) and how such is broken into zones; the potential for flue gas condensation.

john wood_3

Efficiency

It is not the material used in the construction that dictates the efficiency, but the design.

Even the cheapest iron boiler today has about an 80% steady state combustion efficiency. SO; If you size it so it runs constantly, you will acheive its best efficiency. Where they fall down is in the standby losses, whare a hot block sits in the basement losing heat into the home in the summer after making domestic hot water. CI guys would do well to start insulating their products. Also going to balanced flue sealed combustion would help immensely as the hot stack on an open combustion boiler just sits there and sucks heat up the stack in the winter. Not to mention the combustion air ventilation air vents you put in constantly losing heat. (you did put them in, didn't you?) If you actually do a real heat loss, you will find that the largestheat loss on most structures is infiltration and ventilation.

Mike T., Swampeast MO

...so if it runs constantly, you will acheive its best efficiency...

...80% steady state combustion efficiency...

But it's the INSIDE you want at steady state REGARDLESS of what is happening outside! As long as the temperature outside changes a steady-state boiler is IMPOSSIBLE!!!!!

To achieve both of those goals when the outdoor temperature changes either the size of the "fire" must vary or the mass of the emitter must be so great that the boilers' output pales compared to its ability to absorb heat.

Boilerpro_3

Another factor, I suspect (Timmie probably knows this)....

is in atmospheric designs the draft through the boiler changes with the temp. Hotter boiler equals more draft, cooler boiler less. I suspect boilers are designed for optimum air mixture at thier min operating temp (for cast iron about 140F)and begin to add excess air as the temp rises in the boiler, so at very high temps you are pulling alot of excess air through the boiler, recuding the overall efficiency. Just a thought.

Boilerpro

Steve Ebels

This is typical

Old school thinking. It is not true, PERIOD!!! I offer this test to anyone with a combustion analyzer. Start with any cast iron boiler at 120* and take readings while it runs up to 140*. Write them down. Now using the same boiler start at 180* and run it up to 200* checking your efficiency again. You'll see a drop of at least 2%, probably more in combustion efficiency. I have on every single boiler I've ever tested in this manner. Those guys don't have a clue what they are talking about. Why do you think the European standard design temp is limited to 167*?? (and they are going to lower that soon from what I hear)

hydronicsmike

I always understood that...

...most boilers are rated for their efficiency while the Boiler is constantly firing, maintaining a 140°F Supply water temperature, continuous circulation with a 20°F deltaT (120°F Return!). And all the energy (BTUhs) are being put into a building or load. But as the supply water temperature increases to over 140°F, the efficiency will start to drop off. You always end up with higher standby losses when the boiler (or water) temperature is higher.

Glenn, Noel what do you say?

Mike

Boilerpro_3

Mike, not surprised about 120F return....

That's about the min return temp Dunkirk recommends. One of the posts currently here of the wall gives a surface temp of 127F for condensing natural gas. Well if the water inside the boiler is at 120F then the fireside of a cast iron boiler is going to be significantly higher. Cast iron is a pretty good insulator when you look closely, so the temperature gradient from the boiler water to the fireside of the heat exchanger is significant. I understand it grows even greater if the water moves slowly through the boiler because an insulating layer of water develops inside the boiler along the walls of the heat exchanger. Look at the capacities of tube heat exchangers when flow rates are low...they drop off very quickly. (you need turbulent flow in those copper tube boilers). So things to ponder.



Boilerpro

Unknown

As I understand it,....

Steve, Mike, Boilerpro, and others are right on the money.

The steady state test is done with 120° return water and a 20° delta T.

The same boiler, tested for steam, would have a much lower rating.

Noel

Unknown

The AFUE Efficiency testing

is done per the standards that Mike just mentioned. There are some outside parameters that can change all of that however. Most atmospherically vented boilers will not want to be operated in a condition whereby the flow through the boiler is any lower than the dew point of the gasses. For natural gas this would be about 138°F. Because 50% excess air is introduced, this is lowered somewhat to about 127°F. The 167°F supply water temperature would make sense in keeping return water above the dew point.

There are some exceptions to this though. If the flow of water through the boiler could be modulated to always let the boiler stay warm, regardless of system water temperature, then the boiler will be safe from possible condensation. This can be done with variable speed injection with primary-secondary piping or by variable blending. The Revolution boiler uses these principles and can safely handle return water as low as 55°F while acheiving 88% AFUE efficiency. In this case, the boiler will have to maintain a higher limit (210°F) to accomplish this. As you can see, there will be exceptions to the standards, but always keep the boiler hot enough to keep those flue gasses from condensing if the boiler is not intended to be a condensing boiler.

Reducing standby losses also makes sense. This is where a cold start boiler with a device such as any of the Tekmar controls with the ability to do a post purge pump run to drop the boiler temperature to a lower temperature level to reduce standby losses. Many of the multistage boiler controls can now control each boiler and it's injection pump as seperate entities to acheive this. For example, four boilers but an eight stage controller. Hope this helps.

Glenn Stanton

Burnham Hydronics